1. Field of the Invention
This invention relates to a power supply holding device for a photographic camera and more particularly to a the power to the holding device which enables power supply to light measuring, computing and display circuits, etc. to be supplied over a prescribed period of time only when necessary.
2. Description of the Prior Art
The conventional power supply control devices for cameras include the type arranged to supply power to the various circuits such as light measuring, computing and display circuits by having a first switch operated by the first operating stroke of a release button and to actuate the camera by having a second switch operated by the second operating stroke of the release button. A typical example of the conventional power supply control devices of this type is a power supply holding device disclosed in a Japanese patent application No. SHO 53-130828 (Japanese Patent Appl. Laid-Open Publication No. SHO 55-57830). This prior art device includes an electronic timer circuit which holds the above stated light measuring, computing and display circuits, etc. in an energized state over a prescribed period and a third switch which renders the electronic timer circuit operative. The arrangement of the prior art is such that, when the third switch is operated, the electronic timer circuit is actuated in response to the operation of the first switch to permit light measuring, computing and display actions for a prescribed period of time. In this case, however, after the third switch is set into an operative state, there is a probability that the operator might forget to bring the switch back to its original position. Under such a condition, accidental or unintended contact of a finger with a shutter release button, even just momentarily, would cause the electronic timer to operate, resulting in wasteful power consumption. It is also possible that, while the camera is in a travelling bag, travelling vibrations might bring another article contained in the same bag into momentary contact with the shutter release button to cause wasteful power consumption through an accidental operation of the electronic timer. This is the first problem with the prior art arrangement.
In the prior art arrangement, the operation of the electronic timer is arranged to begin at the point of time when the shutter release button is released from the first stroke of a depressing operation thereon. Therefore, in cases where the timer is not required, if the first stroke operation is performed and the third switch is not brought back back to its original position and if, thereafter the shutter button is released from the first stroke operation, the timer, which is not actually necessary at all, is operated for the whole length of the prescribed time, wasting power. This is a second problem with the prior art arrangement.
Further, in the case of the above stated prior art arrangement, once the timer circuit is operated by the first stroke operation, the operation of the timer circuit persists until the end of the timer, operating time even when a second stroke operation on the shutter release button is performed during the operation of the timer circuit. Even after completion of an exposure which follows the second stroke operation, the operation of the timer circuit still continues, thereby wasting power as long as the timer operating time does not comes to an end. This is the third problem with the prior art arrangement.